BOARD AND MANUFACTURE THEREOF

Abstract: Board of lignocellulose-containing material and a method of manufacturing such board. The material is disintegrated to particles or fibres, dried, glued prior to or after the drying, formed to a mat and finally dried by hot pressing to a finished board. According to the invention, the lignin is modified chemically at least in the surface layer of the particles or fibers prior to the final drying, so that an increased binding reactivity is obtained by the introduction of hydrophilic groups and so that the softening temperature of the lignin is lowered. Thereby the lignocellulose material in the board is partially sulphonated so that the organically combined sulphur exceeds the native sulphur content of the material but is lower than 0.4 %.

Description: Board and manufacture thereof This invention relates to board of lignocellulose- conta ning material m the form of particles or fibers such as particle board and fiberboard. The invention also relates to a method of manufacturing such board. Board of this type is manufactured by disintegrating the lignocellulose-contaming material to particles or fibers. This disintegrated material is then dried, glued and formed to a mac. Thereby glueing can be performed prior to or after the drying. The finished board is then produced by final drying of the mat by hot pressing for hardening the glue so that a finished board is formed. Examples of this type of board are particle board and MDF 'Medium Density Fiberboard) . Particle board is made of material, whicn is disintegrated to coarse particles while fiberboard is made of material, which is disintegrated by defibeπng, i.e. to more or less exposed fibers. The strength of the board is determined by the binding of the particles or fibers to each other. The strength of dry-formed glued board is determined by the glue addition, m that the hardened glue binds the particles or fibers togetner. As binding agent at the manufacture of glued board normally urea-formaldehyde glue is used, which is admixed to tne disintegrated material. The amount of glue can be up to 6% or more. The glue is relatively expensive, and, therefore, it is desired to minimize the glue addition. A smaller glue amount, nowever, deteriorates the strength of the finished board. In view thereof, glue addition has to be optimized. The present invention renders it possible to improve the strength of the aforesaid types of board. Alternatively, the glue addition can be reduced while maintaining the strength of the board. According to the invention, the lignm at least m the surface layer of the particles or fibers is modified chemically prior to the final drying, so that an increase m binding reactivity is obtained by introducing hydrophilic groups, and so that the softening temperature of the lignm is lowered. The chemical modification of the lignm can be brought about by chemicals, for example sulpnonation or ozone treatment, or some other way. The invention is described in greater detail in the following, with reference to some embodiments thereof. At the manufacture of fiberboard according to the dry method, by starting with raw material of chips, the chips are disintegrated by defibermg a defibermg apparatus in one or more steps. After the disintegration, glueing, drying, forming and hot pressing are carried out. Alternatively, drying can take place prior to glueing. At hot pressing, a temperature of 100-115°C is tried to be obtained at the centre of the board for glueing systems based on urea-formaldehyde. At this method, chemicals reacting with the lignm are aαded to the material before the hot pressing. The cnemical treatment, for example sulpnonation, modifies the lignm chemically so that an increased binding reactivity is obtained by the introduction of hydrophilic groups. The softening temperature of the lignm also is lowered at the same time. As examples of such chemicals known per se can be mentioned Na 2 S0 3 and NaHSO,. The chemicals can be added after the final defibermg of the material, during or prior to the defibermg. It can be suitable to add the chemicals in direct connection to the defibermg m a d sc refiner, for example in the inlet, the centre or further outward to the periphery of the discs. At defioermg m two or more steps, the chemicals should be added immediately before or αurmg the second or last defibermg step. Other chemical dosing points can be m the blow line after the refiner, m connection with the drying or at the preheating before the defibermg. At the manufacture of particle board, the raw material m the form of chips is disintegrated to particles. The particles are dried, glued and formed to a web, which is hot pressed to a finished board. As in the case of fiberboard manufacture, hardenable glue based on urea-formaldehyde normally is used. Chemicals reacting with the lignm are added also in this case prior to the hot pressing. The chemical treatment has the same object and can be the same as at the manufacture of fiberboard. The chemicals can be added after the final disintegration of the material, during or before the disintegration, preferably immediately before the disintegration, so that the chemical reaction in the lignin takes place during the disintegration and subsequent drying. As stated above, the chemical treatment, preferably sulphonation, has the object to chemically modify the lignin at least in the surface layer of the particles or fibers, partly by introducing hydrophilic groups so that an increased binding reactivity is obtained in the lignin, and partly by lowering the softening temperature of the lignin. The chemically modified surface layer should have a thickness of about lμm where the lowering of the softening temperature shall be at least 10°C. At sulphonation, the lignin should be so sulphonated that a sulphur distribution over the entire cross-section of the particles or fibers is obtained, but where the sulphur content is concentrated to a surface layer of the particles or fibers. Thus, the sulphur content in a 2μm thick surface layer should be at least 50% higher than the average sulphur content in the remaining parts of the particles or fibers. Further, the sulphur content in a lμm thick surface layer should be at least twice as high as the average sulphur content in the remaining parts of the particles or fibers. At the final pressing of the board the temperature is increased above the hardening temperature of the glue in the entire board in order to ensure throughhardening of the board. The lowest temperature in the board at hot pressing, thus, must exceed the hardening temperature required. Therefore, the softening temperature of the lignin in the surface layer of the particles or fibers shall be lowered to a temperature below the lowest temperature in the board at hot pressing. As a result of the lowered softening temperature, the soft lignin layer in the surface of the particles or fibers is pressed out in the point of contact with adjacent particles or fibers, whereby the size of the contact surfaces increases, the glue is utilized more effectively and improved strength properties are obtained, especially in transverse direction of the board. The increase m binding reactivity of the lignm by the introduction of hydrophilic groups further implies, that the lignm can form hydrogen bindings more easily. Thereby the binding forces per surface unit increase, and the strength of the board is increased additionally. Chemical modification of the lignm in the mnner portions of the particles or fibers should be avoided m order to maintain their dimensional stability. Low dosing of sulphite, therefore, is of advantage. Low dosing yields substantially sulphonation of the primary wall and secondary wall of the fiber. At higher dosing, the sulphur distribution correlates well with the lignm distribution m the cell wall. Against this background, sulpnur for the sulphonation shall be added m an amount of at maximum 13 kg sulphur, preferably at maximum 6 kg sulphur, per ton of lignocellulose-contammg material. In order to achieve the desired chemical modification of tne lignm in the surface layer of the particles or fibers, the chemicals should be added at or after the disintegration of the material. The dosing of chemicals, the reaction time and temperature conditions must be accurately controlled. By sulpnonatmg the lignm selectively m the surface layer of the particles or fibers, the modification of the lignm is acnieved at lower dosmgs, and large parts of the interior of the particles or fibers will be unaffected by the sulphonation, so that these parts remain stiff and dimensional stable, which is of aαvantage for the strength of the board. Thereby the lignocellulosic material in the form of particles or fibers is partially sulphonated so that the organically combined sulphur exceeds the native sulphur content of the lignocellulose material but is lower than 0.4%. Preferably the combined sulphur content is 0.04-0.2%. The sulphur is substantially combined with the lignm of the lignocellulose material in the form of sulphonic acid groups. EXAMPLE 1 Raw material in the form of chips from European beech was preheated to 175°C, and an Na 2 S0 3 solution was added. Thereafter disintegration of the material was carried out in a defibering apparatus. The charged amount of sulphite (Na 2 S0 3 ) corresponded to 10 kg/ton chips (2.5 kg sulphur/ton chips) . A reference charge without chemical addition was also made. The softening temperature of the lignin in the reference charge was about 125°C, while the lignin softening temperature of the charge treated with chemical was lowered to about 100°C in the surface layer of the fiber material. In the blow line after the defibering apparatus a urea-formaldehyde glue (UF-glue) was added. The charged amount for material treated with sulphite was corresponded by 8% UF- glue, counted on dry fiber material and for the reference material 13% UF-glue, counted on dry fiber material. Thereafter the two material charges were dried to a dry matter content of about 90% and dry-formed to mats. These fiber mats were pre- pressed with a pressure of 1.5 MPa and hot pressed to board for 6 minutes at 170°C. The temperature at the centre of the board at the end of hot pressing was measured to be between 115 and 120°C, i.e. lower than the softening temperature of the lignin in the reference charge but higher than the softening temperature of the lignin in the charge treated with sulphite. The strength of the board thus manufactured was tested. The results obtained were as follows: Reference Sulphite-treated Property charge charge Bending strength, MPa 39.5 39.0 Internal bond strength MPa 1 .8 1.9 UF-glue content, % 13.0 8.0 The result shows that equivalent strengths for the board was obtained although the glue content in the board with lowered lignin softening temperature was considerably lower. This confirms the technical effect referred to above. EXAMPLE 2 Raw material in the form of chips from spruce was preheated to 170°C, and an Na 2 S0 3 solution was added. The material was thereafter defibered m a defibermg apparatus. The charged amount of sulphite corresponded to 15 and, respectively, 22 kg/ton chips (3.8 and, respectively 5.6 sulphur/ton chips) . A reference charge without chemical addition was also made. No glue addition was made at this test. The softening temperature of the lignm in the reference charge was about 125°C, while the lignm softening temperature of the charge treated with chemical was lowered to below 100°C m the surface layer of the fiber material. After defibermg the fiber material was dried to a dry matter content of about 90% and dryformed to mats. These mats were pre-pressed with a pressure of 1.5 MPa and hot pressed to board for 6 minutes at 170°C. The temperature at the centre of the board at the end of hot pressing was measured to be between 115 and 120°C, i.e. lower than the softening temperature of the lignm m the reference charge but higher than the softening temperature of the lignm in the charge treated with sulphite. The strength of the board thus manufactured was tested. The results obtained were as follows: Property Reference Sulphite 15 kg/ton Sulphite 22 kg/ton Bending strength MPa 5 8 9 0 12 6 Modulus of elasticity MPa 785 1599 1796 The result shows a substantial difference between the properties of the board treated with chemical and of the reference board. The strengths show the effect of increased binding reactivity and increased binding area, which verifies the technical effect of the invention. The invention, of course, is not restricted to the embodiments αescribed, but can oe varied within tne scope of the invention lαea.

Claims: Claims 1. Dry-formed and hot pressed board of lignocellulose- containing material in the form of particles or fibers, c h a r a c t e r i z e d i n t h a t the lignocellulose material is partially sulphonated so that the organically combined sulphur exceeds the native sulphur content of the lignocellulose material but is lower than 0.4%. 2. Board as defined in claim 1, c h a r a c t e r i z e d i n that the organically combined sulphur content is 0.04-0.2%. 3. Board as defined in claim 1-2, c h a r a c t e r i z e d i n that the sulphur is substantially combined with the lignin of the lignocellulose material in the form of sulphonic acid groups. 4. Board as defined in any one of claims 1-3, c h a r a c t e r i z e d i n that the combined sulphur is concentrated to a surface layer of the particles or fibers so that the sulphur content in a 2μm thick surface layer is considerably higher, preferably at least 50% higher, than the average sulphur content in the remaining part of the particles or fibers. 5. Board as defined in claim 4, c h a r a c t e r i z e d i n that the sulphur content in a lμm thick surface layer is at least twice as high as the average sulphur content in the remaining part of the particles or fibers. 6. A method of manufacturing board of lignocellulose- containing material, where the material is disintegrated to particles or fibers, dried, glued prior to or after the drying, formed to a mat and finally dried by hot pressing to a finished board, c h a r a c t e r i z e d i n that prior to the final drying the lignin at least in the surface layer of the particles or fibers is modified chemically, so that an increased binding reactivity is obtained by introducing hydrophilic groups and so that the softening temperature of the lignin is lowered. 7. A method as defined in claim 6, c h a r a c t e r i z e d i n that the softening temperature of the lignin is lowered to a temperature below the lowest temperature required in the board for hardening the glue at the hot pressing. 8. A method as defined in claim 6 or 7, c h a r a c t e r i z e d i n that the lignin is modified chemically by the addition of chemicals reacting with the lignin. 9. A method as defined in claim 8, c h a r a c t e r i z e d i n that the lignin is modified chemically by sulphonation. 10. A method as defined in any one of claims 6-9, c h a r a c t e r i z e d i n that the softening temperature of the lignin is lowered by at least 10°C in a surface layer of about lμm thickness on the particles or fibers. 11. A method as defined in claim 9, c h a r a c t e r i z e d i n that the lignin is sulphonated so that a sulphur distribution across the entire cross-section of the particles or fibers is obtained. 12. A method as defined in claim 11, c h a r a c t e r i z e d i n that the lignin is sulphonated so that the sulphur content in a 2μm thick surface layer is at least 50% higher than the average sulphur content in the remaining part of the particles or fibers. 13. A method as defined in any one of claims * 9-12, c h a r a c t e r i z e d i n that sulphonation of the lignin is made by addition of chemicals containing sulphur in an amount of at maximum 13 kg, preferably at maximum 6 kg sulphur per ton of lignocellulose-containing material. 14. A method as defined in claim 8, c h a r a c t e r i z e d i n that the lignin is modified chemically by ozone treatment. 15. A method as defined in claim 8, c h a r a c t e r i z e d i n that the chemicals are added after the final disintegration of the material. 16. A method as defined in claim 8, c h a r a c t e r i z e d i n that the chemicals are added prior to or during the disintegration. 17. A method as defined in any one of claims 6-16, c h a r a c t e r i z e d i n that the disintegration comprises defibering of the material in one or several steps.

Inventors: LINDSTRUEM LARS-AAKE, MIKAELSSON ARON, OLOFSSON THOMAS, SAEFSTROEM CHRISTER, LINDSTRUEM, LARS-AAKE, MIKAELSSON, ARON, OLOFSSON, THOMAS, SAEFSTROEM, CHRISTER
Applicants: LINDSTRUEM LARS AAKE, MIKAELSSON ARON, OLOFSSON THOMAS, SAEFSTROEM CHRISTER, SUNDS DEFIBRATOR IND AB, LINDSTRUEM, LARS-AAKE, MIKAELSSON, ARON, OLOFSSON, THOMAS, SAEFSTROEM, CHRISTER, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1994
Main classification: B27N1
IPC codes: B27K5, B27N1, B27N3